Method for modifying surface of titanium or titanium alloy by fluidized bed carburization

ABSTRACT

The present invention relates to a method for modifying the surface of titanium or a titanium alloy by a fluidized bed carburization process. The surface hardness of the titanium or the titanium alloy could be improved by the fluidized bed carburization process. The above-mentioned method includes the following steps: providing a fluidized bed furnace, in which a carburizing agent containing charcoal and carbonate powders of which the weight ratio is 10˜8:0.5˜2 is used; placing titanium or a titanium alloy in the fluidized bed furnace at the temperature ranging from 900° C. to 1200° C. for carburizing the titanium metal or the titanium alloy for a period more than 3 minutes; and then taking the titanium metal or the titanium alloy out for quenching.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for modifying the surface of titanium or a titanium alloy and, more particularly, to a method for modifying the surface of titanium or a titanium alloy by a fluidized bed carburization process.

2. Description of Related Art

Commercial titanium alloys have been widely applied in military and aerospace industries over the last ten years since titanium and titanium alloys have many advantages, such as inert chemical properties, high temperature resistance, excellent corrosion resistance and high strength-to-weight ratio. Currently, titanium alloys are being further developed in leisure and bio-pharmaceutical industries to manufacture products including sport equipment, sports cars, surgical implant materials and so on. The development of surface treating technology to improve the near surface properties such as wear resistance and resistance to halide ions attack of titanium alloys has been extensive and progressive.

It was suggested that the molten salt carburization (MSC) process performed in the surface modification of ferrous alloys can be used for the surface modification of commercial purity titanium (CP-Ti) and Ti-6A1-4V alloys (Ti-64), such that the specimens treated with the molten salt carburization (MSC) process can achieve the purposes of surface modification and hardening. However, the molten salt carburization (MSC) provides only limited improvements in surface hardness and hardened depth of Ti-6Al-4V alloy.

Therefore, it is desirable to provide a process that can improve commercial purity titanium and Ti-6Al-4V alloy more than the molten salt carburization process to enhance the surface hardness of titanium alloys and thereby to expand the application thereof.

SUMMARY OF THE INVENTION

The present invention provides a method for modifying the surface of titanium or a titanium alloy by a fluidized bed carburization process. By the fluidized bed carburization process, a mixture of diamond, graphite and rutile can be formed on the surface of the titanium or the titanium alloy so as to improve the surface hardness of the titanium and the titanium alloy. Accordingly, the titanium and the titanium alloy treated with the fluidized bed carburization process have more superior surface hardness to those subjected to the molten salt carburization process. The method includes the following steps: providing a fluidized bed furnace, in which a carburizing agent containing charcoal and carbonate salt powder of which the weight ratio is 10˜8:0.5˜2 is used; placing titanium or a titanium alloy in the fluidized bed furnace at the temperature ranging from 900° C. to 1200° C. for carburizing the titanium or the titanium alloy for a period more than 3 minutes; and then taking the titanium or the titanium alloy out for quenching.

In the above-mentioned method, the fluidized bed furnace can further contain alumina powder. The alumina powder is mixed with the charcoal and the carbonate salt powder. The alumina fine powder can allow the temperature at the fluidized bed to be uniform and thereby the carburization in the fluidized bed furnace is performed more uniformly.

In the above-mentioned method, the carbonate salt powder can be any kind of carbonate salt powder that can produce carbon monoxide by thermal decomposition. Preferably, the carbonate salt powder is selected from the group consisting of magnesium carbonate powder, calcium carbonate powder, strontium carbonate powder, barium carbonate powder and a mixture thereof. Herein, the carbonate salt powder is used to produce carbon monoxide during heating, such that titanium or titanium alloys can be surrounded uniformly by carbon monoxide to perform carburization. More preferably, barium carbonate powder is used in the fluidized bed furnace.

In the above-mentioned method, the charcoal can be any kind of carbon source powder, as long as it has excellent purity and no impurity that badly influences the carburization process. For example, Acacia Koa charcoal provides carbon with high purity and thereby can be used in the present invention. Preferably, the size of charcoal used in the present invention is in a range from 6 to 12 mesh, and more preferably, in a range from 8 to 10 mesh. If the size of charcoal is too large, the reaction cannot be performed uniformly. In addition, preferably, the weight ratio of charcoal to carbonate salt powder is 9:1.

In the above-mentioned method, the fluidized bed furnace can be filled with an inert gas as carrier gas, such that there is steady airflow in the fluidized bed furnace to favor the performance of carburization. In general, preferably, the circular flow of the inert gas is controlled in a range from 2 to 4 l/min. If the circular flow of the inert gas is too low, carburization cannot be performed uniformly. If the circular flow of the inert gas is too large, the produced carbon monoxide will be diluted and thereby the carburization reaction is slowed down. Preferably, the circular flow of the inert gas is 3 l/min.

In addition, the temperature of the carburizing agent used in the fluidized bed furnace influences the carburization reaction. If the temperature is too high, the metal specimen may become molten. If the temperature is too low, the performance of carburization is poor. Accordingly, preferably, the temperature is controlled in a range from 900 to 1200° C. More preferably, the temperature is controlled in a range from 930 to 1000° C. Also, the reaction time influences the carburization reaction, and thereby the reaction time has to be more than 3 minutes. Preferably, the predetermined time is in a range from 5 to 30 minutes to achieve the purpose for modifying the surface hardness of titanium and titanium alloys.

In conclusion, the present invention uses the fluidized bed carburization process to modify the surface hardness of titanium or titanium alloys. By the method, carbon deficient titanium carbide (TiC_(1-x)) is formed at the near surface of titanium or titanium alloys, and a mixture of diamond, graphite and rutile is formed on the outermost surface of titanium or titanium alloys. Accordingly, the method disclosed in the present invention can enhance surface hardness, surface wear resistance and resistance to halide ions attack of titanium or titanium alloys, so as to obtain titanium or titanium alloys with more improved surface hardness.

Other objects, advantages, and novel features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1( a) shows a surface image from scanning electron microscopy of CP-Ti not treated with a fluidized bed carburization process according to Comparison Example 1 of the present invention;

FIG. 1( b) shows a surface image from scanning electron microscopy of Ti-6Al-4V not treated with a fluidized bed carburization process according to Comparison Example 2 of the present invention;

FIG. 1( c) shows a surface image from scanning electron microscopy of CP-Ti treated with a fluidized bed carburization process according to Example 1 of the present invention;

FIG. 1( d) shows a surface image from scanning electron microscopy of CP-Ti treated with a fluidized bed carburization process according to Example 2 of the present invention;

FIG. 1( e) shows a surface image from scanning electron microscopy of CP-Ti treated with a fluidized bed carburization process according to Example 3 of the present invention;

FIG. 1( f) shows a surface image from scanning electron microscopy of CP-Ti treated with a fluidized bed carburization process according to Example 4 of the present invention;

FIG. 1( g) shows a surface image from scanning electron microscopy of Ti-6Al-4V treated with a fluidized bed carburization process according to Example 5 of the present invention;

FIG. 1( h) shows a surface image from scanning electron microscopy of Ti-6Al-4V treated with a fluidized bed carburization process according to Example 6 of the present invention;

FIG. 1( i) shows a surface image from scanning electron microscopy of Ti-6Al-4V treated with a fluidized bed carburization process according to Example 7 of the present invention; and

FIG. 1( j) shows a surface image from scanning electron microscopy of Ti-6Al-4V treated with a fluidized bed carburization process according to Example 8 of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Examples 1˜8

First, commercial purity titanium (CP-Ti) or a Ti-6Al-4V alloy (Ti-64) is received in mill-annealed condition and provided as a raw material. The components contained in the raw material are shown in Table 1, in which the unit is wt % unless otherwise stated.

TABLE 1 Fe C O N H Al V Ti CP-Ti 0.02 0.10 0.02 0.01 68 ppm — — bal. Ti-64 0.18 0.01 0.127 0.010 43 ppm 6.0 3.9 bal.

The above-mentioned raw material is cut into a specimen of 40×14×1 mm and then ground with 1000 grade emery paper in running water to avoid oxide film forming on the surface of the raw material specimen as such film influences carburization. Herein, the surface roughness is measured by Talysurf 6 system (Rank Taylor-Hobson) and the results are given in Table 2.

TABLE 2 Condition R_(a) (μm) R_(q) (μm) R_(t) (μm) R_(tm) (μm) CP-Ti as-received 0.227 0.287 1.85 1.45 abraded 0.075 0.091 0.63 0.52 Ti-64 as-received 0.467 0.578 3.51 3.09 abraded 0.081 0.097 0.81 0.73 R_(a): The arithmetic mean of the departure of the profile from the mean line R_(q): The root-mean-square parameter corresponding to R_(a) R_(t): The maximum peak to valley height of the profile in the assessments R_(tm): The mean of all the R_(t) values obtained in the assessment

A fluidized bed furnace is provided, in which an appropriate amount of alumina powder is contained to allow the temperature in the fluidized bed furnace to be uniform so as to perform carburization uniformly during carburization. Subsequently, the above-mentioned abraded raw material and a carburizing agent are placed in the fluidized bed furnace. The carburizing agent contains Acacia Koa charcoal and barium carbonate of which the weight ratio is 9:1, and the moisture contained in Acacia Koa charcoal and the barium carbonate is removed by nitrogen backpressure to favor the subsequent fluidization. Meanwhile, an inert gas such as Argon is used in the fluidized bed furnace as a carrier gas, and the flow rate thereof is controlled in 3 l/min. The condition of gas in the fluidized bed furnace throughout the process is monitored by gas chromatography. After the carburizing process, the carburized specimens are water quenched to room temperature. The processing parameters are given in Table 3.

TABLE 3 Example Raw material Processing parameters 1 CP-Ti 930° C. for 5 minutes and then water quenched 2 CP-Ti 930° C. for 30 minutes and then water quenched 3 CP-Ti 1000° C. for 5 minutes and then water quenched 4 CP-Ti 1000° C. for 30 minutes and then water quenched 5 Ti-64 930° C. for 5 minutes and then water quenched 6 Ti-64 930° C. for 30 minutes and then water quenched 7 Ti-64 1000° C. for 5 minutes and then water quenched 8 Ti-64 1000° C. for 30 minutes and then water quenched

Comparison Example 1

As Example 1, CP-Ti is used but not treated with the fluidized bed carburization process.

Comparison Example 2

As Example 5, Ti-6Al-4V is used but not treated with the fluidized bed carburization process.

Analysis of Examples and Comparison Examples

The surface morphology and the microstructure of the specimens treated or not are observed by scanning electron microscopy (SEM). In addition, the Vickers hardness number (VHN) of the specimens treated or not is measured by MVK-H100 (Akashi).

FIGS. 1( a) and (b) show surface morphology images from scanning electron microscopy of CP-Ti and Ti-6Al-4V not treated with a fluidized bed carburization process according to Comparison Example 1 and Comparison Example 2, respectively.

FIGS. 1( c), (d), (e) and (f) show images from scanning electron microscopy of CP-Ti specimens with a mixture of diamond, graphite and rutile formed on the surface thereof after a fluidized bed carburization process according to Examples 1, 2, 3 and 4, respectively.

FIGS. 1( g), (h), (i) and (j) show images from scanning electron microscopy of Ti-6Al-4V specimens with a mixture of diamond, graphite and rutile formed on the surface thereof after a fluidized bed carburization process according to Examples 5, 6, 7 and 8, respectively.

In view of FIGS. 1( c) to (f), it can be known that CP-Ti has excellent surface uniformity after undergoing the fluidized bed carburization process at 930° C. or 1000° C. for 5 or 30 minutes.

Also, FIGS. 1( g) to (j) show that Ti-6Al-4V has excellent surface uniformity after undergoing the fluidized bed carburization process at 930° C. or 1000° C. for 5 or 30 minutes.

The data obtained by a micro hardness tester are shown in Table 4.

TABLE 4 Specimens Vickers hardness number (VHN) Comparison Example 1 257 Comparison Example 2 352 Example 1 882 Example 2 1032 Example 3 1072 Example 4 1270 Example 5 905 Example 6 1047 Example 7 1156 Example 8 1283

Comparing Examples 1 to 4 with Comparison Example 1, it can be found that CP-Ti treated with a fluidized bed carburization process at 930° C. or 1000° C. for 5 or 30 minutes can have hardness up to 1270 VHN, i.e. the increase of hardness is 1013 VHN. In addition, comparing Examples 5 to 8 with Comparison Example 2, it can be found that Ti-6Al-4V treated with a fluidized bed carburization process at 930° C. or 1000° C. for 5 or 30 minutes can have hardness up to 1283 VHN, i.e. the increase of hardness is 931 VHN.

Accordingly, the surface hardness of titanium or titanium alloys can be improved significantly by a fluidized bed carburization process. The efficiency provided by the fluidized bed carburization process is better than that provided by a conventional molten salt carburization (MSC) process. Thereby, in comparison with a conventional molten salt carburization (MSC) process, the fluidized bed carburization process according to the present invention requires shorter time and can improve further the surface hardness of titanium or titanium alloys.

Although the present invention has been explained in relation to its preferred embodiment, it is to be understood that many other possible modifications and variations can be made without departing from the scope of the invention as hereinafter claimed. 

1. A method for modifying the surface of titanium or a titanium alloy by a fluidized bed carburization process, comprising: providing a fluidized bed furnace, in which a carburizing agent containing a charcoal and carbonate salt powder of which the weight ratio is 10˜8:0.5˜2 is used; placing the titanium or the titanium alloy in the fluidized bed furnace at the temperature ranging from 900° C. to 1200° C. for carburizing the titanium or the titanium alloy for a period more than 3 minutes; and taking the titanium or the titanium alloy out for quenching.
 2. The method as claimed in claim 1, wherein alumina powder is used in the fluidized bed furnace and mixed with the charcoal and the carbonate powder.
 3. The method as claimed in claim 1, wherein the carbonate salt powder is selected from the group consisting of magnesium carbonate powder, calcium carbonate powder, strontium carbonate powder, barium carbonate powder and a mixture thereof.
 4. The method as claimed in claim 1, wherein the carbonate salt powder is barium carbonate powder.
 5. The method as claimed in claim 1, wherein the charcoal is Acacia Koa charcoal.
 6. The method as claimed in claim 1, wherein the size of the charcoal ranges from 6 to 12 mesh.
 7. The method as claimed in claim 1, wherein the size of the charcoal ranges from 8 to 10 mesh.
 8. The method as claimed in claim 1, wherein an inert gas is used in the fluidized bed furnace as a carrier gas with a circular flow of 2˜4 l/min.
 9. The method as claimed in claim 8, wherein the circular flow of the inert gas is 3 l/min.
 10. The method as claimed in claim 1, wherein the temperature of the carburizing agent ranges from 930 to 1000° C.
 11. The method as claimed in claim 1, wherein the period ranges from 5 to 30 minutes.
 12. The method as claimed in claim 1, wherein the weight ratio of the charcoal to the carbonate salt powder is 9:1. 